Povey, Holma, and Toskala, in xe2x80x9cTDD-CDMA Extension to FDD-CDMA Based Third Generation Cellular System,xe2x80x9d 1997 IEEE 6th Int""l Conf. on Universal Personal Communications Record, San Diego, Oct. 12-16, 1997, pages 813-17, describe a third-generation FDD-CDMA mobile radio system which has, within one transmission frame, an asymmetric transmission capacity in the uplink and in the downlink direction between a base station and a subscriber.
In radio communications systems, information (for example voice, picture information or other data) is transmitted with the aid of electromagnetic waves via a radio interface between a transmitting and a receiving radio station (base station and mobile station, respectively). The electromagnetic waves are thereby radiated at carrier frequencies which are located within the frequency band provided for the respective system. In the case of GSM (Global System for Mobile Communications), the carrier frequencies are in the range of 900, 1800 and 1900 MHz, respectively. For future mobile radio networks with CDMA or TD/CDMA transmission methods via the radio interface, for example the UMTS (Universal Mobile Telecommunication System) or other 3rd-generation systems, frequencies are provided within the frequency band of approx. 2000 MHz.
During their propagation in a propagation medium, signals are subject to interference due to noise. Signal components pass through different propagation paths due to diffraction and reflection and become superimposed at the receiver. Such a multi-path channel is described by a channel pulse response. In the case where there are a number of signal sources, these signals also become superimposed. To distinguish between the signal sources and thus to evaluate the signals, frequency-division multiple access (FDMA), time-division multiple access (TDMA), or a method known as code-division multiple access (CDMA) are used.
A special instance of the time-division multiple access (TDMA) is a time-division duplex (TDD) transmission method in which the transmission takes place in a common narrowband frequency channel both in the uplink direction (i.e., from the mobile station to the base station) and in the downlink direction (i.e., from the base station to the mobile station). Such a TDD transmission method is known from the DECT (digital enhanced cordless telephony) system. In the DECT system, one frame consists of 24 time slots, half of which are continuously used in each case for the uplink and the downlink, respectively.
Unlike mobile radio networks, the DECT system does not have an allocation of radio engineering resources by the network but the mobile stations look for their own frequency range and time slot in accordance with the transmission conditions. All bursts transmitted in both directions of transmission have a common length which is greatly shortened in comparison with the duration of a time slot so that the bursts can be easily separated at the receiving end even under adverse transmission conditions and for the time slots around the switching time between the uplink and downlink. However, the spectral efficiency is not very good since the unused period of a time slot is lost for transmitting information.
It is accordingly an object of the invention to provide an improved method and an improved radio communication system for transmitting information which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type, and in which the spectral efficiency is increased.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of transmitting information between a base station and other radio stations in a radio communications system, which comprises, within an individual transmission frame:
transmitting information with first bursts in a frequency channel in a downlink;
transmitting information with second bursts in the frequency channel in an uplink;
establishing at least one switching time between the uplink and the downlink;
transmitting a third burst of shortened duration compared with a duration of at least one of the first bursts and the second bursts adjacent the switching time within the transmission frame, i.e., before and/or after the switching time.
In other words, the information is transmitted by first bursts in a frequency channel in the downlink and by second bursts in the same frequency channel in the uplink. At least one switching time (instant) is established in this case between the downlink and uplink. These first and second bursts can be configured for the highest possible spectral efficiency in accordance with the rules of mobile radio telephony. However, a third burst of shortened duration compared with the first and second bursts is transmitted before and/or after the switching time. This prevents the receiving and transmitting operation from overlapping for the switching time due to disadvantageous relative signal propagation times. Thus, the data rate only needs to be reduced for one or two bursts per frame in comparison with the maximum data rate of the remaining bursts so that the achievable data rate, and thus the spectral efficiency, is improved overall.
In accordance with an added feature of the invention, the first, second, and third bursts are transmitted in time slots of uniform period. There is thus a fixed time-slot pattern which can be used by the base station and mobile stations for reference with respect to the transmitting times. With such a time-slot structure, it is important that the bursts arrive at the receiver, especially at the base station, in the correct time slot around the switching time. Thus, in particular, the switching time which brings about the switching from transmitting to receiving for the base station must be protected by the shortened third bursts. It is assumed that the mobile station is synchronized to the base station.
In accordance with an additional feature of the invention, a time difference between the duration of the first bursts and the third bursts is set to approximately a round-trip delay between the base station and the other radio station.
In accordance with another feature of the invention, a time difference between the duration of the first and second bursts and a duration of a time slot within the transmission frame substantially corresponds to a duration of a channel pulse response.
These adjustments result in the best possible utilization of a time slot so that high data rates can be achieved and, nevertheless, no collisions occur between transmitting and receiving operation even under disadvantageous transmission conditions.
In accordance with a further feature of the invention, the shortened third burst is transmitted approximately at the switching time from downlink to uplink.
For radio communication systems comprising radio cells of variable extent, the duration of the third burst is advantageously set to be inversely proportional to the radius of a radio cell of the base station. Thus, the loss of data rate is less for small radii than for large radii. This further increases the spectral efficiency particularly in radio communication systems comprising macrocells or hierarchical cell structures, respectively.
In accordance with again an added feature of the invention, the frame is formed of a number of bursts and the switching time is shiftable in either direction of transmission within a frame. This is especially advantageous because it provides for an asymmetric distribution of the data rate in the uplink and downlink in accordance with the instantaneous demand. For data transmission services, e.g. mobile World Wide Web browsers, the volume of information to be transmitted will frequently be greater in the downlink than in the uplink. This can be achieved by shifting the switching time to favor the downlink whilst retaining good spectral efficiency. If at a later time an increased data rate is again needed in the uplink, e.g. due to a voice transmission or due to large volumes of data to be transmitted in the uplink, which need a symmetric resource distribution or a resource distribution favoring the uplink, the switching time can be adapted to this.
The novel method is particularly advantageous to TDD systems in which the frequency channels have a wide bandwidth and a number of signals which can be distinguished by means of CDMA codes are simultaneously transmitted in one frequency channel.
In the case of broadband time slots, optimum utilization of the frequency channel along the time axis is of particular importance.
With the above and other objects in view there is also provided, in accordance with the invention, a radio communications system, comprising:
a base station for transmitting information to at least one other radio station;
a device for allocating radio engineering resources, and for defining a transmission of information, within one transmission frame, by first bursts in a frequency channel in a downlink and by second bursts in the same frequency channel in an uplink;
a controller in one of the base station and the at least one other radio station, the controller being configured to
establish at least one switching time between the downlink and uplink; and
to set a third burst within the frame adjacent the switching time (i.e., before and/or after the switching instant), the third burst being shorter than at least one of the first and second bursts, and being transmitted within the same transmission frame as the first and second bursts.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and radio communication system for transmitting information between a base station and other transceiver stations, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.